1. Field
This disclosure relates to a method for preparing a membrane electrode assembly (MEA) using a low-temperature transfer method, an MEA prepared thereby, and a fuel cell using the same.
2. Background of the Related Art
A polymer electrolyte membrane fuel cell (PEMFC) generates electricity through electrochemical reaction of fuel such as hydrogen, methanol, etc. (on the anode side) and air (on the cathode side). Since the PEMFC may operate continuously for a long period of time as long as the fuel is supplied, it may have advantages over the secondary cells that it does not have problems of limited capacity, inconvenience due to charging, or the like.
One of the key elements of PEMFC is a membrane electrode assembly (MEA).
Methods for preparing MEA may be classified into catalyst-coated substrate (CCS) method and catalyst-coated membrane (CCM) method.
In CCS methods, a catalyst layer may be formed on a porous gas diffusion substrate to prepare an anode and a cathode. Then, after positioning a polymer electrolyte membrane between the anode and the cathode, high pressure and temperature is applied to prepare an MEA.
In CCM methods, a catalyst layer may be directly coated on a polymer electrolyte membrane to prepare an MEA. It is known that the CCM method may provide better performance and endurance than the CCS method, in general.
The CCM methods may include brushing, screen printing, tape casting, spraying, transferring, etc.
The transfer method is known to be advantageous in large-scale production over other methods. High-temperature transferring has been often used for the transfer method.
In the high-temperature transfer method, a catalyst layer may be coated on a transfer substrate made of a non-ion-conductive polymer film such as Teflon, Kapton, etc., and high temperature of about 200° C. and high pressure of about 10 MPa may be applied to transfer the catalyst layer from the transfer substrate to a polymer electrolyte membrane. This method may be advantageous in performance and long-term operation property because a thin catalyst layer can be obtained and the contact resistance between the catalyst layer and the polymer electrolyte membrane may be lower as compared to MEAs prepared otherwise.
The high-temperature transfer method may require a high temperature of about 150° C. or above, i.e., a temperature higher than the glass transition temperature of the polymer electrolyte membrane. For reference, a usual transfer temperature in the high-temperature transfer method may be about 180° C. or more. If the temperature is lower than about 180° C., the catalyst layer may not be well transferred.
In order to prevent deformation of the polymer electrolyte membrane at high temperature, an H+-type polymer electrolyte membrane may need to be substituted to have Na+-type, TBAOH+-type, etc., which substitution can raise the glass transition temperature. The MEA may be substituted again to have H+-type by immersing the MEA in aqueous sulfuric acid solution after transfer of the catalyst layer.